“Virtual substrates” based on silicon (Si) and germanium (Ge) provide a platform for new generations of VLSI devices that exhibit enhanced performance when compared to devices fabricated on bulk Si substrates. The important component of a SiGe virtual substrate is a layer of SiGe that has been relaxed to its equilibrium lattice constant (i.e., one that is larger than that of Si). This relaxed SiGe layer can be directly applied to a Si substrate (e.g., by wafer bonding or direct epitaxy) or atop a relaxed graded SiGe layer, in which the lattice constant of the SiGe material has been increased gradually over the thickness of the layer. The SiGe virtual substrate can also incorporate buried insulating layers, in the manner of a silicon-on-insulator (“SOI”) wafer. To fabricate high-performance devices on these platforms, thin strained layers of Si, Ge, or SiGe are grown on the relaxed SiGe virtual substrates. The resulting biaxial tensile or compressive strain alters the carrier mobilities in the layers, enabling the fabrication of high-speed devices, or low-power devices, or both.
A technique for fabricating strained Si wafers includes the following steps:                1. Providing a silicon substrate that has been edge polished;        2. Epitaxially depositing a relaxed graded SiGe buffer layer to a final Ge composition on the silicon substrate;        3. Epitaxially depositing a relaxed SiGe cap layer having a uniform composition on the graded SiGe buffer layer;        4. Planarizing the SiGe cap layer by, e.g., chemical mechanical polishing (“CMP”);        5. Epitaxially depositing a relaxed SiGe regrowth layer having a uniform composition on the planarized surface of the SiGe cap layer; and        6. Epitaxially depositing a strained silicon layer on the SiGe regrowth layer.        
The deposition of the relaxed graded SiGe buffer layer enables engineering of the lattice constant of the SiGe cap layer (and therefore the amount of strain in the strained silicon layer), while reducing the introduction of dislocations. The lattice constant of SiGe is larger than that of Si, and is a direct function of the amount of Ge in the SiGe alloy. Because the SiGe graded buffer layer is epitaxially deposited, it will initially be strained to match the in-plane lattice constant of the underlying silicon substrate. However, above a certain critical thickness, the SiGe graded buffer layer will relax to its inherently larger lattice constant.
The process of relaxation occurs through the formation of misfit dislocations at the interface between two lattice-mismatched layers, e.g., a Si substrate and a SiGe epitaxial layer (epilayer). Because dislocations cannot terminate inside a crystal, misfit dislocations have vertical dislocation segments at each end, i.e., threading dislocations, that may rise through the crystal to reach a top surface of the wafer. Both misfit and threading dislocations have stress fields associated with them. As explained by Eugene Fitzgerald et al., Journal of Vacuum Science and Technology B, Vol. 10, No. 4, 1992, incorporated herein by reference, the stress field associated with the network of misfit dislocations affects the localized epitaxial growth rate at the surface of the crystal. This variation in growth rates may result in a surface cross-hatch on lattice-mismatched, relaxed SiGe buffer layers grown on Si.
The stress field associated with misfit dislocations may also cause dislocation pile-ups under certain conditions. Dislocation pile-ups are a linear agglomeration of threading dislocations. Because pile-ups represent a high localized density of threading dislocations, they may render devices formed in that region unusable. Inhibiting the formation of dislocation pile-ups is, therefore, desirable.
Dislocation pile-ups are formed as follows. (See, e.g., Srikanth Samavedam et al., Journal of Applied Physics, Vol. 81, No. 7, 1997, incorporated herein by reference.) A high density of misfit dislocations in a particular region of a crystal will result in that region having a high localized stress field. This stress field may have two effects. First, the stress field may present a barrier to the motion of other threading dislocations attempting to glide past the misfits. This pinning or trapping of threading dislocations due to the high stress field of other misfit dislocations is known as work hardening. Second, the high stress field may strongly reduce the local epitaxial growth rate in that region, resulting in a deeper trough in the surface morphology in comparison to the rest of the surface cross-hatch. This deep trough may also pin threading dislocations attempting to glide past the region of high misfit dislocations. This cycle may perpetuate itself and result in a linear region with a high density of trapped threading dislocations, i.e., dislocation pile-up.
The term “MOS” (meaning “metal-oxide-semiconductor”) is here used generally to refer to semiconductor devices, such as FETs, that include a conductive gate spaced at least by an insulting layer from a semiconducting channel layer. The terms “SiGe” and “Si1-xGex” are here used interchangeably to refer to silicon-germanium alloys. The term “silicide” is here used to refer to a reaction product of a metal, silicon, and optionally other components, such as germanium. The term “silicide” is also used, less formally, to refer to the reaction product of a metal with an elemental semiconductor, a compound semiconductor or an alloy semiconductor.
One challenge to the manufacturability of MOS devices with strained layers is that one or more high temperature processing steps are typically employed after the addition of the strained material. This can cause intermixing of the strained layer and underlying material. This intermixing is generally referred to as interdiffusion, and it can be described by well-known diffusion theory (e.g., Fick's laws). One example of interdiffusion is found in a FET where a strained layer is used as the channel. In this example, one or more impurities (e.g., dopants) are implanted after addition of the strained layer. If implantation is followed by a moderately high temperature step (e.g., a drive-in or anneal step), there can be rampant interdiffusion of the channel by the implant impurity due to the presence of implant damage and excess point defects in the strained layer. A result is that the impurity is present in the strained layer. Stated differently, the impurity profile (i.e., a gradient describing the impurity concentration as a function of location in the overall semiconductor or device) has a non-zero value in the strained layer. Presence of one or more impurities in the strained layer can, at certain concentrations, degrade overall device performance.
From the foregoing, it is apparent that there is still a need for a way to produce semiconductor structures and devices that include one or more strained layers that are not subject to the incursion of one or more impurity species during structure or device fabrication.